Light and Color
Problems How do cameras measure light and color? Radiometry How do humans perceive light and color? Photometry How do computers represent light and color? How do monitors display light and color?
Intensity Perception of intensity is nonlinear Perceived brightness Amount of light
Modeling Nonlinear Intensity Response Brightness (B) usually modeled as a logarithm or power law of intensity (I) B = k log I B B = I 1/ 3 Exact curve varies with ambient light, adaptation of eye I
Adelson s Checker Illusion
Adelson s Checker Illusion
CRT Response Power law for Intensity (I) vs. applied voltage (V) γ I = V γ = 2.5 Other displays (e.g. LCDs) contain electronics to emulate this law
Digression: Monitor Knobs Brightness knob is offset Contrast knob is scale I = contrast ( V + brightness) γ Yes, the names are misleading
Cameras Original cameras based on Vidicon obey power law for Voltage (V) vs. Intensity (I): γ V = I γ = 0.45 Vidicon + CRT = almost linear!
CCD Cameras Camera gamma codified in NTSC standard CCDs have linear response to incident light Electronics to apply required power law So, pictures from most cameras (including digital still cameras) will have γ = 0.45
Consequences for Vision Output of most cameras is not linear Know what it is! (Sometimes system automagically applies gamma correction ) Necessary to correct raw pixel values for: Reflectance measurements Shape from shading Photometric stereo Recognition under variable lighting
Consequences for Vision What about e.g. edge detection? Often want perceptually significant edges Standard nonlinear signal close to (inverse of) human response Using nonlinear signal often the right thing
Contrast Sensitivity
Contrast Sensitivity Contrast sensitivity for humans about 1% 8-bit image (barely) adequate if using perceptual (nonlinear) mapping Frequency dependent: contrast sensitivity lower for high and very low frequencies
Contrast Sensitivity Campbell-Robson contrast sensitivity chart
Bits per Pixel Scanned Pictures 8 bits / pixel / color 6 bits / pixel / color Marc Levoy / Hanna-Barbera
Bits per Pixel Scanned Pictures (cont.) 5 bits / pixel / color 4 bits / pixel / color Marc Levoy / Hanna-Barbera
Bits per Pixel Line Drawings 8 bits / pixel / color 4 bits / pixel / color Marc Levoy / Hanna-Barbera
Bits per Pixel Line Drawings (cont.) 3 bits / pixel / color 2 bits / pixel / color Marc Levoy / Hanna-Barbera
Seurat: The Side Show, 1888
Color
Aguilonius, 1613
Newton: color circle from Optiks, 1704
Johann Lambert: Color pyramid, 1772
Runge: Colour Sphere, 1809
Modern Understanding of Color Two types of receptors: rods and cones Rods and cones Cones in fovea
Rods and Cones Rods More sensitive in low light: scotopic vision More dense near periphery Cones Only function with higher light levels: photopic vision Densely packed at center of eye: fovea Different types of cones color vision
Electromagnetic Spectrum Visible light frequencies range between... Red = 4.3 x 10 14 hertz (700nm) Violet = 7.5 x 10 14 hertz (400nm) Figures 15.1 from H&B
Visible Light Color may be characterized by Hue = dominant frequency (highest peak) Saturation = excitation purity (ratio of highest to rest) Lightness = luminance (area under curve) White Light Figures 15.3-4 from H&B Orange Light
Color Perception Spectral-response functions of the three types of cones. Tristimulus theory of color Figure 13.18 from FvDFH
Tristimulus Color Any distribution of light can be summarized by its effect on 3 types of cones Therefore, human perception of color is a 3-dimensional space Metamerism: different spectra, same response Color blindness: fewer than 3 types of cones Most commonly L cone = M cone
Color Models RGB XYZ CMY HSV etc
Color Models Different ways of parameterizing 3D space RGB Official standard: R = 645.16 nm, G = 526.32 nm, B = 444.44 nm Most monitors are some approximation to this
Color CRT Figure 2.8 from H&B
Color LCD
RGB Color Model Colors are additive Plate II.3 from FvDFH R G B Color 0.0 0.0 0.0 Black 1.0 0.0 0.0 Red 0.0 1.0 0.0 Green 0.0 0.0 1.0 Blue 1.0 1.0 0.0 Yellow 1.0 0.0 1.0 Magenta 0.0 1.0 1.0 Cyan 1.0 1.0 1.0 White 0.5 0.0 0.0? 1.0 0.5 0.5? 1.0 0.5 0.0? 0.5 0.3 0.1?
RGB Color Cube Figures 15.11&15.12 from H&B
RGB Spectral Colors Amounts of RGB primaries needed to display spectral colors? Figure 15.5 from H&B
XYZ Color Model (CIE) Amounts of CIE primaries needed to display spectral colors Figure 15.6 from H&B
XYZ Colorspace RGB can t represent all pure wavelengths with positive values Saturated greens would require negative red XYZ colorspace is a linear transform of RGB so that all pure wavelengths have positive values
CIE Chromaticity Diagram
CIE Chromaticity Diagram Normalized amounts of X and Y for colors in visible spectrum (White) Figure 15.7 from H&B
CIE Chromaticity Diagram Compare Color Gamuts Identify Complementary Colors Determine Dominant Wavelength and Purity Figures 15.8-10 from H&B
RGB Color Gamut Color gamut for a typical RGB computer monitor Figure 15.13 from H&B
CMY Color Model Colors are subtractive C M Y Color 0.0 0.0 0.0 White 1.0 0.0 0.0 Cyan 0.0 1.0 0.0 Magenta 0.0 0.0 1.0 Yellow 1.0 1.0 0.0 Blue 1.0 0.0 1.0 Green 0.0 1.0 1.0 Red 1.0 1.0 1.0 Black 0.5 0.0 0.0? 1.0 0.5 0.5? 1.0 0.5 0.0? Plate II.7 from FvDFH
CMY Color Cube Figure 15.14 from H&B
HSV Color Model H S V Color 0 1.0 1.0 Red 120 1.0 1.0 Green 240 1.0 1.0 Blue * 0.0 1.0 White * 0.0 0.5 Gray * * 0.0 Black 60 1.0 1.0? 270 0.5 1.0? 270 0.0 0.7? Figure 15.16&15.17 from H&B
Colorspaces for Television Differences in brightness more important than differences in color YC r C b, YUV, YIQ colorspaces = linear transforms of RGB Lightness: Y=0.299R+0.587G+0.114B Other color components typically allocated less bandwidth than Y
Perceptually-Uniform Colorspaces Most colorspaces not perceptually uniform MacAdam ellipses: color within each ellipse appears constant (shown here 10X size)
Perceptually-Uniform Colorspaces u v space u' = v' = X X 4X + 15Y 9Y + 15Y + 3Z + 3Z Not perfect, but better than XYZ
L*a*b* Color Space Another choice: L*a*b* = = = 1/ 3 1/ 3 1/ 3 1/ 3 1/ 3 200 * 500 * 16 116 * n n n n n Z Z Y Y b Y Y X X a Y Y L
L*a*b* Color Space Often used for color comparison when perceptual differences matter
Summary Perception and representation of Intensity, frequency, color Color Tristimulus theory of color CIE Chromaticity Diagram Different color models
Preattentive Processing Some properties are processed preattentively (without need for focusing attention). Important for art, design of visualizations what can be perceived immediately what properties are good discriminators what can mislead viewers Preattentive processing sildes from Healey http://www.csc.ncsu.edu/faculty/healey/pp/pp.html
Example: Color Selection Viewer can rapidly and accurately determine whether the target (red circle) is present or absent. Difference detected in color.
Example: Shape Selection Viewer can rapidly and accurately determine whether the target (red circle) is present or absent. Difference detected in form (curvature)
Pre-attentive Processing < 200 250 ms qualifies as pre-attentive eye movements take at least 200ms yet certain processing can be done very quickly, implying low-level processing in parallel If a decision takes a fixed amount of time regardless of the number of distractors, it is considered to be preattentive
Example: Conjunction of Features Viewer cannot rapidly and accurately determine whether the target (red circle) is present or absent when target has two or more features, each of which are present in the distractors. Viewer must search sequentially.
Example: Emergent Features Target has a unique feature with respect to distractors (open sides) and so the group can be detected preattentively.
Example: Emergent Features Target does not have a unique feature with respect to distractors and so the group cannot be detected preattentively.
Asymmetric and Graded Preattentive Properties Some properties are asymmetric a sloped line among vertical lines is preattentive a vertical line among sloped ones is not Some properties have a gradation some more easily discriminated among than others
SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC
Text NOT Preattentive SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM GOVERNS PRECISE EXAMPLE MERCURY SNREVOG ESICERP ELPMAXE YRUCREM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC SUBJECT PUNCHED QUICKLY OXIDIZED TCEJBUS DEHCNUP YLKCIUQ DEZIDIXO CERTAIN QUICKLY PUNCHED METHODS NIATREC YLKCIUQ DEHCNUP SDOHTEM SCIENCE ENGLISH RECORDS COLUMNS ECNEICS HSILGNE SDROCER SNMULOC
Preattentive Visual Properties [Healey 97] length Triesman & Gormican [1988] width Julesz [1985] size Triesman & Gelade [1980] curvature Triesman & Gormican [1988] number Julesz [1985]; Trick & Pylyshyn [1994] terminators Julesz & Bergen [1983] intersection Julesz & Bergen [1983] closure Enns [1986]; Triesman & Souther [1985] colour (hue) Nagy & Sanchez [1990, 1992]; D'Zmura [1991] Kawai et al. [1995]; Bauer et al. [1996] intensity Beck et al. [1983]; Triesman & Gormican [1988] flicker Julesz [1971] direction of motion Nakayama & Silverman [1986]; Driver & McLeod [1992] binocular lustre Wolfe & Franzel [1988] stereoscopic depth Nakayama & Silverman [1986] 3-D depth cues Enns [1990] lighting direction Enns [1990]
Accuracy Ranking of Quantitative Perceptual Tasks Estimated; only pairwise comparisons have been validated [Mackinlay 88 from Cleveland & McGill]
Visual Illusions People don t perceive length, area, angle, brightness they way they should Some illusions have been reclassified as systematic perceptual errors e.g., brightness contrasts (grey square on white background vs. on black background) partly due to increase in our understanding of the relevant parts of the visual system Nevertheless, the visual system does some really unexpected things
Illusions of Linear Extent Mueller-Lyon (off by 25-30%) Horizontal-Vertical
Illusions of Area Delboeuf Illusion Height of 4-story building overestimated by approximately 25%